People just like to hate on the USA.
When the American crowd is dull and quite they are bashed. Yet the dull and quite Japanese crowd is praised.
When Americans chant 'USA USA' they are bashed and called drunk rednecks. When Brazilians, Australians, Canadians, Mexicans, etc chant their nationalistic slogans they are praised for being enthusiastic about the fight.
You can't win with these bigots.
America is the top dog and people always hate the top dog. The Persians, Greeks, Romans, Mongolians, British, and every other hegemonic power were hated during their time. It's natural. The reason is because the hegemonic power provokes inferiority in all the second-rate states. Everyone likes to believe their own culture or country is in some way special or superior, and when someone else is doing better it stings the self-image. The feeling of inferiority leads to insecurity, insecurity leads to bitterness, and bitterness leads to irrational hate.
For the last 70 years America has been the global hegemon that has disproportionately dominated every aspect and industry in the world. Everyone revolves their life around American creations. Look at yourselves right now -- you're probably on an American computer, run by an American operating system, on an American website, discussing an American MMA organization.
America is in the spotlight more than other nations and thus gets more hate. Nobody puts energy into hating second-rate countries because no one cares about second-rate countries. No one pays much attention to what they do. Their 'flaws' are not put under a microscope like America's 'flaws.'